1. Technical Field
The present invention relates to a method for manufacturing an electronic device comprising overmolded MEMS devices.
The invention also relates to an electronic device comprising overmolded MEMS devices.
The invention particularly, but not exclusively, relates to a method for manufacturing an electronic device comprising MEMS sensors mounted on an LGA substrate, wherein the MEMS sensor needs a physical interface to communicate with the outside of the electronic device and the following description is made with reference to this field of application by way of illustration only.
2. Description of the Related Art
As it is well known, a MEMS device (micro-electro-mechanical system) is a miniaturized device or, in any case, a device having micrometric size which integrates mechanical and electrical functions in a chip or die of semiconductor material, for example of silicon, and which is realized by using micro-manufacturing lithographic techniques. The final assembled device is typically made of the silicon die wherein the MEMS device is integrated and, optionally, of integrated circuits for specific applications mounted on a substrate, for example of the LGA or BGA type (Land Grid Array or Ball Grid Array), flanked to or piled onto the MEMS device, by using conventional assembling processes.
A cover or cap, fixed to the substrate, overmolds the MEMS device and the other devices mounted on the substrate, forming a casing which protects the MEMS device against external physical stresses.
If the MEMS device is a pressure, gas, liquid sensor or a microphone, the cover is provided with a window to allow the interaction between the device and the outside of the assembled device.
It is also known that the substrate of the LGA/BGA type is formed by conductive layers insulated from each other by means of layers of insulating or dielectric material. The conductive layers are shaped in conductive tracks insulated from each other by layers of insulating or dielectric material. Conductive through-holes, called “vias”, typically realized through the insulating layers according to an orthogonal orientation with respect to the insulating layers, are provided to form conductive paths between conductive tracks belonging to different conductive layers.
The MEMS devices are then electrically connected with the outside of the final device through wires connecting contact pads provided on the MEMS devices with conductive tracks present on the substrate inside the cover.
A solution of this type is described for example in the PCT patent application, with publication number WO 2007/042336, filed on Apr. 26, 2006 by the assignee of the present application. In this patent application a MEMS device, in particular a pressure sensor, is realized on a substrate of the LGA type, to which it is glued, through a layer of epoxy glue. This sensor has a cavity above which there is a membrane and is connected to the substrate through metallic conductive wires. It is then covered by a closing wafer equipped with an opening in correspondence with the membrane of the sensor and through which the sensor is in communication with the outside. All the device is finally overmolded in a casing.
Although advantageous under several aspects, this solution shows different drawbacks. In fact, the complete device is made by first realizing a casing bottom, then the different components are affixed to the casing bottom, and finally the casing is molded and the element to control the sensor is inserted through the window of the casing. For these devices, moreover, the procedure of alignment and positioning of the window to introduce the element for controlling the sensor is rather complicated, making the realization of the device difficult to be reproduced. Moreover, the manufacturing process of these devices is complicated by the presence of different assembling steps and relatively expensive.
A second solution, described in U.S. Patent Application Publication No. 2002/0070464 to the assignee of the present application, provides the use of a casing which, by using a conventional technique, is equipped with a window in correspondence with an integrated electronic device, for example a sensor housed inside the casing and which must be put in communication with the outside of the casing. This window is obtained by using the same mold which is used to realize the casing; this mold is equipped with a protrusion projecting internally in correspondence with the sensor. After having fixed the sensor and a relative control circuitry to a semiconductor substrate, which serves as support, a surface of the sensor is covered with a coating layer formed by material of the elastic type. The substrate is inserted in the mold so that the protrusion is in correspondence and in contact with the coating layer. The mold is then filled in by injection with an insulating material to realize, in a single step, the casing with window.
Although meeting the aim, also this solution is not exempt from drawbacks, such as the reproducibility of the process, the stability of the shape of the dispensed elastic material, the reliability and the strength of the device which can, in fact, be subject to delaminations at the interface between the insulating material and the material of the elastic type.
Another technique for manufacturing a MEMS device overmolded in a casing uses a molding machine commercially known as “film assisted mold,” which realizes the cap of the device thanks to a polymeric film, interposed between the mold and the device itself, which allows to expose the silicon in a remarkably controlled way.
The disadvantage of this solution consists in that the increase of the size of the holes made on the cap determines a weakening of the silicon slice which serves as cap, and a subsequent breakage of this one during the completion of the device.
Moreover, to ease the interaction between a MEMS device, for example a pressure sensor, and the fluid outside the casing overmolding it methods have been implemented for manufacturing micro-channels buried in the MEMS device, below the membrane or active element. A method which realizes buried micro-channels of this type is described in the patent application, with publication number U.S. 2006/0260408, filed on May 4, 2006 by the assignee of the present application.
A second method known for the formation of micro-channels is described in the U.S. Patent Application Publication No. 2006/0246416. According to this method, the micro-channels are formed in the substrate of a first “chip”, called micro-porous “chip”, which is then glued to a second “chip”, called micro-fluidic “chip”. A third example of formation of micro-channels is described in the U.S. Patent Application Publication No. 2005/0151244, wherein micro-channels are formed first in a “cooling plate” using to cool an electronic “chip”.
It is known to realize micro-channels in a silicon substrate through the combination of suitably shaped layers.
Another aspect to be taken into account in the manufacturing of the MEMS devices, in particular in the ultra thin ones, or for “package” applications, is the use of photo-sensitive resins (“photo-resist”) being very thick and with high “aspect ratio” (i.e.: with high ratio between width and height of the device, also known as “aspect ratio”), so as to obtain vertical walls in relatively high structures with a good control of the size on the whole height.
A known technique to obtain structures with high “aspect ratio” with sub-micron resolution in very thick “photo-resist” is the X-ray lithography, used in the LIGA process (“Lithography, Galvanoforming and Abformung”) to form very thick layers of PMMA (polymethylmethacrylate). However, the cost of the manufacturing of a device, made by using the LIGA process with X-ray lithography, is strongly influenced by the high cost of the X-ray source (synchrotron radiation) and by the complex technology of the masks.
Recently, instead, a new type of “photo-resist”, having characteristics similar to the PMMA and having the possibility of being used in a process of the LIGA type, is used for the applications of ultrathin MEMS with high “aspect ratio”.
The characteristics of this new type of “photo-resist”, called SU-8, are described in the publications “A Novel Fabrication Method of Embedded Micro Channels Employing Simple UV Dosage Control and Antireflection Coating”, F. G. Tseng, Y. J. Chuang and W. K. Lin, 2002 IEEE; and “High-Aspect-Ratio, Ultrathick, Negative-Tone Near-UV Photoresist for MEMS Applications,” M. Despont, H. Lorenz, N. Fahrni, J. Brugger, P. Renaud and P. Vettiger, 1997 IEEE.
The SU-8 is a “photo-resist” similar to the epoxy resin, sensitive to the radiation near the UV and based onto the resin EPON SU-8 (from “Shell Chemical”). The fundamental characteristic, which makes the SU-8 useful for the ultrathick “photo-resist” applications, is its very low optical absorption in a range of radiations near the UV, which determines uniform exposure conditions according to the thickness, allows to form perfectly vertical walls and to have a good size control on the height of the whole formed structure. Another advantage of the SU-8 is its capacity of self-planarization during the “prebake” and then to eliminate the “edge-bead” effect, determining a good contact between the mask and the “photo-resist” in the contact lithography.
As reported in the second one of the above publications, it has been proved that, with a coating having single SU-8 layer, thicknesses can be obtained, in a reproducible way, of more than 500 μm and that even thicker “photo-resist” can be obtained through multiple coatings, up to 1200 μm of thickness with a double-layer coating. The “aspect ratio” found for structures exposed to the radiation near the UV (400 nm) can be greater than 18 and remains constant for a thickness comprised between 80 and 1200 μm.